The Gulf of Maine Program has need of bathymetric information to characterize
inshore and wetland habitats for water birds, shorebirds, and fishes. The
distribution of sounding data available for deeper areas is sufficient to
generate a relatively coarse resolution bathymetry grid. We have need of
finer spatial resolution around inshore features, but soundings of intertidal
areas are not abundant. As an alternative, I elected to supplement bathymetric
data with tidal exposure classifications described from aerial photography
(e.g., "regularly flooded, irregularly exposed, irregularly flooded") in
the USFWS National Wetland Inventory data sets, and also channels and tidal
flats mapped from photography in Maine's Coastal Marine Geologic Environments
(CMGE) and Massachusetts orthophoto wetlands maps. These classes were not
translated into absolute elevations, but instead were coded as to position
within the intertidal zone, and the general cover type (e.g., high marsh,
lower intertidal flats, etc.), which may convey useful ecological information.
The mapping area includes the U.S. portion of the Gulf of Maine: eastern
coastal Massachusetts, coastal New Hampshire, and Maine.

USGS hydrologic features; polygons of these digital maps were coded with
NWI attributes (based on NWI paper maps), to fill in areas without digital
NWI or orthophoto wetlands (part of Massachusetts).

Surveys by the Jackson Estuarine Laboratory, original data obtained from
Dr. Carl Friedrichs of Virginia Institute of Marine Science. Also, soundings
obtained from the U.S. Army Corps of Engineers for the Piscataqua River and
for Hampton Harbor; used forNew Hampshire.

NOAA bathymetric grids (30 meter) of New England embayments at:
http://mapfinder.nos.noaa.gov/;
these high quality grids, were used to supplement or replace other subtidal
bathymetry information; used for Massachusetts.

NOAA nautical charts were used as background for checking sources. Also,
I digitized some line and point features for use in New Hampshire and Maine.
I also used rasterized versions of charts to 'cut and paste' some channel
and inlet depth zones where these were an improvement over the interpolation
from limited point or line data. Arcs digitized from NOAA metric charts were
provided by R. Kelley/OGIS, for use in Maine.

Data from all sources was converted to depth in feet reference mean low water,
and projected as coverages to UTM.

TINs (triangulated irregular network; ESRI, ArcInfo) were created from the
following data sets:

Subtidal bathymetry components:

Mean low water (0 depth line): The intertidal (0) line was the outer boundary
of the intertidal grid, based on aerial photo-interpreted this was supplemented
with arcs digitized from NOAA charts and USGS quad sheets.

Mean high water (elevation varies along coast): Used the USGS coast line
coverage as mhw; made the depth value +19' at Calais, 16' at West Quoddy
Head, 15 at Moose Cove, etc, based on tide table: 'Rise for substations of
Portland'; set at + 6 to + 7 for Massachusetts and New Hampshire.

Soundings: NOAA points, from CDROM, < 30 meters only; used all Jackson
Laboratory and Corps of Engineers New Hampshire soundings;

USGS and NOAA arcs.

COMBINING THE INTERMEDIATE PRODUCTS:

I first created TINs which were used to produce grids of nearshore bathymetry
from the above sources, at 15 m and at 30 m resolution. The USGS 15 - second
grid was used to fill in deeper areas; the resultant grid was overlain with
the NOAA embayment data where this more closely corresponded with NOAA charts.
Finally, the intertidal grids were superimposed.

Subtidal bathymetry of certain embayments and channels, mostly on the Maine
coast, still was found a poor match with the published NOAA charts. This
was because the TINS were based on limited digital data sets for these areas,
while the charts were based on far more complete soundings. Using scanned
versions of the charts, I converted the images to grids. I then used the
color rendition as the link to the depth range designated on the chart, and
clipped out replacement 'patches' for each of the embayments to be corrected.
The NOAA chart soundings were inspected, and a intermediate depth value was
assigned each patch. Patches and base grid were combined by retaining the
deeper value from either source.

Two final grids were produced: one retained all integer depth values, and
a "descriptive" attribute to group intertidal features and depths into a
few classes (see below). The other grid has only the classes, and therefore
has a smaller file size.